Resolve VPN client hostnames

I have a number of VPN clients connecting to a Ubuntu 12.04 server (This was set up using Zentyal 3.0). I need to connect many more.
I would like to be able to address the clients from the server by client hostname.
The Openvpn server gives all the clients IPs in the 196.168.160.0/24 range. The Server running Openvpn has a 192.168.1.20 address.
The DNS server is up and running but it does not resolve the host names of the clients on the VPN.

How can I set up the DNS on the server so that I can ping, ssh etc., and connect to the Clients by hostname, from the server, rather than by IP address.
Thanks,
Jodel